1. Field of the Invention
This invention relates generally to the field of high frequency integrated circuits.
2. Description of the Related Art
High frequency microwave-range integrated circuits (ICs) based on gallium arsenide (GaAs) are known. Lower frequency-range integrated circuits which include components fabricated on silicon (Si) are also known. It would be highly desirable to extend the high frequency performance capacity of GaAs-based ICs to Si-based ICs, using materials and equipment which are adaptations of those whose use is well established in silicon processing.
Silicon technology has been the foundation of the microelectronics industry, but in attempts to extend the more mature silicon technology to the integration of high frequency microwave components such as coplanar transmission lines and inductors, the inherent limitation of the resistivity of silicon, which is maximal in pure silicon, has been a barrier to fabricating Si-based devices which are technically noncompetitive with GaAs in the microwave range. For example, loss at about 10 GHz on silicon is approximately 20 times that on GaAs; GaAs-based microwave structures have the low loss tangent that would be desirable to achieve in manufacturable Si-based structures. Manufacturability requires processes and results that are stable, predictable, reproducible and cost effective. High frequency devices based on GaAs are relatively more expensive to fabricate than are lower frequency devices based on Si, but lower frequency devices based on Si can be fabricated by processes that produce stable, predictable and reproducible results.
U.S. Pat. No. 5,528,209 issued Jun. 18, 1996 to Mcdonald et al. describes a silicon-based high frequency monolithic structure in which the high frequency transmission lines are fabricated by electroplating gold. Gold plating may give rise to problems such as cost, added process steps to create barriers to electromigration of gold into copper and handling and disposal of the gold electroplating baths and rinses. The present invention does not include gold processing. Rather, wet electroprocessing is avoided by using sputter deposition and sputter cleaning and ashing. The '209 patent describes via fabrication by reactive ion etching (RIE). In the present invention, via (through-hole) photolithography, including wet or reactive ion etching (RIE) is avoided in fabricating internal vias by using laser ablation, a process which provides superior control of the critical via dimension of slope angle. The '209 patent uses benzocyclobutene (BCB) resin, which was found not to laser well for the purposes of the present invention. The polyimide (PI) used in the present invention has the advantages of lasering well, low dielectric constant, low moisture absorbency, ability to be applied and cured in a layer up to at least 15 microns thick, and ability to withstand the temperature required to solder or wire bond the completed device without cracking. A paper presented at the 1995 IEEE conference, "High Performance Microwave Elements for SiGe MMICs" by Michael Case et al. describes a Si-based microwave device using BCB, a resinous composition used in the prepreg art, as the dielectric material. BCB was found to be unsuitable for the processing of the present invention, particularly with respect to laser processing.
An article by Anthony Cataldo and Ron Wilson beginning on page 1 in the Electronic Engineering Times dated Jan. 26, 1988 describes some of the IBM activity in the area of SiGe-based RF ICs.
"Low-Loss Microwave Transmission Lines and Inductors Implemented in a Manufacturable Si/SiGe HBT Process" by David C. Laney, Lawrence E. Larson, John Malinowski, David Harame, Seshu Subanna, Rich Volant, Michael Case and Paul Chan was orally presented in September, 1998 at the BCTM meeting in Minnesota. In it are described experimental results of measurements made for square planar inductors and microstrip transmission lines for standard Si VLSI structures having CuAl metallization and thick polyimide dielectric. The work indicates the manufacturability in Si VLSI technology of these lines and inductors and predicts their use in high performance, low cost Si-based 5-10 GHz MMICs in the future.
U.S. Pat. No. 4,830,706 issued May 16, 1989 to Ronald S. Horwath et al. describes one method, not involving laser, in which slope-walled vias with rounded corners are fabricated by finally curing a resinous insulating material in which, after a preliminary partial cure, the via walls had been conventionally straight and corners square. The patent describes problems associated with straight-walled vias and benefits of slope-walled vias and rounding at the intersection of via wall and planar surface.
None of the references anticipates the process and article of manufacture of the present invention.